The present disclosure relates generally to laser emission spectroscopy and, more particularly, to an apparatus and system for in situ laser plasma spectroscopy analysis.
The elemental composition of solids can be determined rapidly and simply with the use of laser plasma spectroscopy (LPS). Also known as laser induced breakdown spectroscopy (LIBS) and laser-induced plasma spectroscopy (LIPS) (as well as other names), LPS uses a high peak power laser pulse to form a microplasma or spark on a sample to be analyzed. A small amount of the sample material is vaporized and a plasma is formed, with the emitting species (e.g., ions, atoms and molecules) in the plasma being identified by spectrally and temporally resolving the plasma light.
Although LPS measurements may be performed remotely by pointing the laser beam directly on the sample in free space, it is an undesirable technique to be used by people working in the field. In an alternative method of remote LPS spectroscopy analysis, the laser output is coupled to an optical fiber that transmits the laser energy to a generally inaccessible location. A probe or borescope then directs the laser energy to the sample surface. In this type of apparatus, a beam splitter is typically inserted inside the probe to separate the incident laser beam from the returning LPS signal, which is in turn coupled to a fiber optic connected to a spectrometer.
Conventional probe designs utilize at least two fibers inside the probe package itself, one for the laser delivery and the other one for LPS signal collection. However, this type of design makes the probe bulky, which can be unsuitable in certain field applications. In other probe designs, two fibers are bundled together as a pair. Laser energy exiting from the laser delivering fiber is focused on the surface of the sample by optics that are also used to image the plasma radiation generated on the sample surface back to the laser delivering fiber. Since the plasma has finite dimension and the optics have aberrations associated therewith, the image of the plasma on the side of fiber is extended, and thus the second fiber next to the laser delivering fiber also collects some LPS generated radiation. However, the collection is secondary and the efficiency is therefore low. Alternatively, other approaches have utilized a curved mirror on the side of the laser-focusing lens to reflect the plasma radiation to a collecting chamber. Again, these types of systems that feature multiple optical devices within the probe assembly itself are very bulky.
Accordingly, it would be desirable to be able to have a field suitable, portable LPS system that can be moved from location to location and can also withstand a harsh environment, featuring (among other aspects) a compact probe that can be held by hand, inserted into and directed by a guide tube, or carried by a robot to a desired location.